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Arrangements in Modern Periodic Table    (Long Form of Periodic Table)    

 The arrangement  in periodic table is given by Bohr so it is also known as Bohr’s table or Long form of periodic table.
This table is prepared on the basis of –

(a) increasing order of atomic number or electronic configuration. (placed in a period)
(b) same number of valence electron/electrons. (placed in one group)

The modern periodic table has 18 vertical columns called groups and seven horizontal rows called periods.


The 18 vertical columns are named or numbered as 1,2,3,…18. Elements of each group has same number of valence electrons, so has similar properties.

    (a) Normal elements or representative elements

All the inner shells are completely filled, only the outermost shell is incomplete. It includes group1
(alkali metal), 2  (alkaline earth metal)  and  group 13 to 17.

    (b) Transition elements

The outermost shell as well as second outermost shell (next to the outermost shell-penultimate shell) are incomplete. It includes elements from group 3 to group12.

    (c) Inner Transition elements or Rare earth elements –

The last three shells (outermost shell and two shells next to outermost shells) are incomplete.

      (i) Lanthanoids– Elements having atomic numbers from 58 to 71 are called lanthanoids and the series is called Lanthanide series. It is placed separately below the main table.

      (ii) Actinoids– Elements having atomic numbers from 90 to 103 are called actinoids and the series is called actinide series. It is also placed separately in the bottom of main table with lanthanide series.

    (d) Nobel gases or Inert gases

Elements of group -18 have fully occupied valence shells are called inert gases or nobel gases.

Read More –

Periodic Classification of Elements – Introduction, Dobereiner’s Traids

Newlands’ Law of Octaves

Mendeleev’s Periodic Table

Importance of Mendeleev’s Periodic Table

Introduction to Modern Periodic Table


The seven horizontal rows are named as period 1,2,3… 7. The number of period shows how many shells are present. For e.g. elements of period 1 have one shell (K) only.

The elements of modern periodic table are roughly divided into metals (left side of the table) and non-metals (right side of periodic table). In the middle of metals and non-metals ,there are some elements called metalloids which are placed diagonally [Boron(B), Silicon(Si), Germanium(Ge), Arsenic(As), Antimony(Sb), Tellurium(Te) and Polanium(Po)].  The properties of metalloids are also intermediate to metals and non-metals.

  • In a period , all elements have different electronic configurations, so have different properties.
  • In a group , all elements have similar electronic configurations or same valence electrons, so have
    similar properties.
  • The period number of an element is equal to the number of shells in its atoms. For e.g. An element belongs to 3rd period , has 3 shells (K, L, M).

Importance and Limitations of Modern Periodic Table


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